Packet networks transport data from a source location to a destination location by organizing the data into self-contained units called packets. Each packet carries its routing information as it passes through a series of routing nodes on its way to the destination location. Each routing node reads the routing information associated with the packet and uses that information to decide the correct path to use to forward the packet. In traditional IP (Internet Protocol) networks, the routing information is made up of individual addresses of source and destination nodes. In more advanced MPLS (Multiprotocol Label Switching) networks, packets are assigned additional labels that group them according to their intermediate or final destinations. This label assignment promotes efficient scaling and quality-of-service assignment in the MPLS networks.
Optical labels can also be useful in circuit-switched networks, especially those capable of dynamically re-routing signals on a wavelength-by-wavelength basis. Typical optical labels are encoded using amplitude-shift keying (ASK) formats, with a particular emphasis on ON-OFF keying (OOK). However, as data rates increase and modulation techniques change (e.g., QPSK and DP-QPSK modulation), OOK can no longer be applied.